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Microsoft Internet Explorer calls telnet.exe with unsafe command-line arguments

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M-024: Microsoft Internet Explorer calls telnet.exe with unsafe
command-line arguments

[CERT Vulnerability Note VU#952611]

November 30, 2001 01:00 GMT
 PROBLEM:           The telnet client can be invoked with unsafe options
                    by arbitrary HTML pages.
 PLATFORM:          IE 6.0 and earlier, when used with the telnet client
                    in services for Unix 2.0.
 DAMAGE:            Remote attackers can execute commands by spawning
                    telnet with a log file option on the command line and
                    writing arbitrary code into an executable file.
 SOLUTION:          Apply patch as described below.
 VULNERABILITY      The risk is HIGH. A remote attacker could write files
 ASSESSMENT:        to the system of an unsuspecting user; including an
                    executable file with malicious intent.

   CIAC BULLETIN:          http://www.ciac.org/ciac/bulletins/m-024.shtml
   ORIGINAL BULLETIN:      http://www.kb.cert.org/vuls/id/952611

[***** Start CERT Vulnerability Note VU#952611 *****]

Vulnerability Note VU#952611

Microsoft Internet Explorer (IE) calls telnet.exe with unsafe command-line
arguments ("Telnet Invocation")


A telnet client can be invoked with unsafe options by arbitrary HTML ("web")
pages when rendered by affected Microsoft Internet Explorer clients.

I. Description

This vulnerability is also known as the "telnet logging" or "telnet
invocation" or "Microsoft IE Telnet Client File Overwrite" vulnerability.
A similar vulnerability was first described in Microsoft Security Bulletin
MS01-015. When vulnerable versions of Internet Explorer render HTML with
malicious 'telnet:' tags, the telnet.exe client can be called with unsafe
arguments. A telnet option may be specified which directs transcripts or
logs of a telnet session to overwrite or create arbitrary files on a
victim's system. These files could contain data chosen by an attacker
to gain further privileges on a target system.

The telnet package used may be installed as part of the Services for
UNIX 2.0 package on computers that are running Windows NT 4.0,
Windows 2000, or Windows XP.

II. Impact

A remote attacker may write files onto the system of a user who either
visited a malicious web site or opened malicious HTML-formatted mail.
The vulnerability could be used to place a file onto the victim's
computer which could then run automatically each time the user logged on
(i.e., a file installed in the victim's Start folder can allow arbitrary
commands to be executed).

III. Solution

Apply the patch provided by Microsoft at


Note this patch can only be applied after having upgraded to Internet Explorer
5.5 Service Pack 2 (SP2), Internet Explorer 5.01 SP2, or having applied the
patch fixing the original vulnerability, Q28043.

No previous updates are needed to patch and remove this vulnerability in
Internet Explorer 6 clients. Alternately, remove the telnet client installed
as part of the "Services for Unix 2.0" package, if installed.

Systems Affected

Vendor      Status        Date Updated
Microsoft   Vulnerable    28-Nov-2001


CERT Advisory CA-2001-21 discusses separate issues involving buffer
overflows in implementations of the telnet daemon.



This variant of the IE "Telnet Invocation" vulnerability first discussed
in CVE-2001-0150 was discussed in Microsoft Security Bulletin MS01-051.

This document was written by Jeffrey S. Havrilla.

Other Information

Date Public 10/10/2001
Date First Published 11/28/2001 11:52:36 PM
Date Last Updated 11/29/2001
CERT Advisory
CVE Name CAN-2001-0667
Metric 30.37
Document Revision 43

[***** End CERT Vulnerability Note VU#952611 *****]

CIAC wishes to acknowledge the contributions of CERT Coordination Center
for the information contained in this bulletin.
CIAC services are available to DOE, DOE Contractors, and the NIH. CIAC can
be contacted at:

    Voice:          +1 925-422-8193 (7 x 24)
    FAX:            +1 925-423-8002
    STU-III:        +1 925-423-2604
    E-mail:          ciac@llnl.gov
    World Wide Web:  http://www.ciac.org/
                     (same machine -- either one will work)
    Anonymous FTP:   ftp.ciac.org
                     (same machine -- either one will work)

This document was prepared as an account of work sponsored by an agency of
the United States Government. Neither the United States Government nor the
University of California nor any of their employees, makes any warranty,
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